linux-kernel - Re: [PATCH v2] pci: lock the pci_cfg_wait queue for the consistency of data

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Date:   Tue, 19 Nov 2019 14:23:05 -0600
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Xiang Zheng <zhengxiang9@...wei.com>
Cc:     willy@...radead.org, wangxiongfeng2@...wei.com,
        wanghaibin.wang@...wei.com, guoheyi@...wei.com,
        yebiaoxiang@...wei.com, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org, rjw@...ysocki.net,
        tglx@...utronix.de, guohanjun@...wei.com, yangyingliang@...wei.com
Subject: Re: [PATCH v2] pci: lock the pci_cfg_wait queue for the consistency
 of data

On Tue, Nov 19, 2019 at 09:15:45AM +0800, Xiang Zheng wrote:
> Commit "7ea7e98fd8d0" suggests that the "pci_lock" is sufficient,
> and all the callers of pci_wait_cfg() are wrapped with the "pci_lock".
> 
> However, since the commit "cdcb33f98244" merged, the accesses to
> the pci_cfg_wait queue are not safe anymore. A "pci_lock" is
> insufficient and we need to hold an additional queue lock while
> read/write the wait queue.
> 
> So let's use the add_wait_queue()/remove_wait_queue() instead of
> __add_wait_queue()/__remove_wait_queue(). Also move the wait queue
> functionality around the "schedule()" function to avoid reintroducing
> the deadlock addressed by "cdcb33f98244".

Procedural nits:

  - Run "git log --oneline drivers/pci/access.c" and follow the
    convention, e.g., starts with "PCI: " and first subsequent word is
    capitalized.

  - Use conventional commit references, e.g., 7ea7e98fd8d0 ("PCI:
    Block on access to temporarily unavailable pci device") and
    cdcb33f98244 ("PCI: Avoid possible deadlock on pci_lock and
    p->pi_lock")

  - IIRC you found that this actually caused a panic; please include
    the lore.kernel.org URL to that report.

You can wait for a while to see if there are more substantive comments
to address before posting a v3.

> Signed-off-by: Xiang Zheng <zhengxiang9@...wei.com>
> Cc: Heyi Guo <guoheyi@...wei.com>
> Cc: Biaoxiang Ye <yebiaoxiang@...wei.com>
> ---
> 
> v2:
>  - Move the wait queue functionality around the "schedule()" function to
>    avoid reintroducing the deadlock addressed by "cdcb33f98244"
> 
> ---
> 
>  drivers/pci/access.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/pci/access.c b/drivers/pci/access.c
> index 2fccb5762c76..09342a74e5ea 100644
> --- a/drivers/pci/access.c
> +++ b/drivers/pci/access.c
> @@ -207,14 +207,14 @@ static noinline void pci_wait_cfg(struct pci_dev *dev)
>  {
>  	DECLARE_WAITQUEUE(wait, current);
>  
> -	__add_wait_queue(&pci_cfg_wait, &wait);
>  	do {
>  		set_current_state(TASK_UNINTERRUPTIBLE);
>  		raw_spin_unlock_irq(&pci_lock);
> +		add_wait_queue(&pci_cfg_wait, &wait);
>  		schedule();
> +		remove_wait_queue(&pci_cfg_wait, &wait);
>  		raw_spin_lock_irq(&pci_lock);
>  	} while (dev->block_cfg_access);
> -	__remove_wait_queue(&pci_cfg_wait, &wait);
>  }
>  
>  /* Returns 0 on success, negative values indicate error. */
> -- 
> 2.19.1
> 
>